The use of mounting substrates in the field of electronics has been known since the development of the transistor. With the development of more complex devices having more functional capability, these substrates have become more important. For example, the substrates now serve as a median for attaching any number of devices such as transistors, diodes, semiconductors, resistors, etc. Generally, a substrate carrying an electronic device is assembled through any number of steps. Solders may be used to attach a device to a board by heat application in any number of processes known to those skilled in the art. In wave soldering, the heat is applied to the mass soldering or printed wiring assemblies on a plastic laminated board. Hundreds of connections can be soldered in a few seconds. In oven soldering, the solder is placed on the assembly and the units are placed in an oven for soldering. Vapor-phase reflow soldering may also be used for mass soldering applications.
Solders may comprise any number of constituents depending upon their application such as a flux. A solder flux generally removes surface compounds, reduces surface tension of molten solder alloy, and deters oxidation. Most inorganic fluxes are a combination of salts and acids dissolved in water with a wetting agent. Most fluxes are organic water insoluble rosins or water soluble organic acids. Rosin fluxes are preferably inert and nonconductive in the cold solid state, but active in removing tarnish when hot. Water soluble organic fluxes are used increasingly in electromechanical and electronic soldering. Most solder alloys are combinations of flux with tin and lead made in many shapes and forms such as for example wire, bar, foil, spears, ring, or paste.
After soldering, flux residues and oxides may be present in the assembly. The residual effects of processing may introduce by-products not acceptable within the resulting electronic device assemblies. Residues of soldering may comprise any number of constituents which may lead to corrosion such as lead and tin carbonates or lead and tin chloride and fluorides. These residues may generally appear as a white residue on the assembly. Solder residues may be corrosive and, if allowed to remain on the assembly, may cause malfunction.
Once assembled, the boards are placed within their environments of use. Depending upon the application, the environment of use may prove hostile to the assembly. Once in use, the assembly may be subject to attack through a number of mechanisms during its lifetime. Residues of processing, including metal chlorides, are activated by humidity and carbon dioxide to create hydrochloric acid or sulfuric acid and, under certain conditions, hydrofluoric acid and nitric acid. Vinyl acetates (by-products of the assembly processes as well) are transformed into corrosive compounds.
One means of alleviating destruction of printed wiring boards, circuit card assemblies as well as any other electronic device or assembly is the use of post processing washing processes. Removal of acidic substances, alkaline substances as well as other matters such as excess heavy metals as a means of reducing the combined effects of these residual constituents in the promotion of corrosion is a concern in the industry.
Halogenated cleaning agents are well known including chlorinated agents such as 1,1,1-trichloroethane and fluorocarbons such as freon. For example, Kraus et al, U.S. Pat. No. 3,799,803, disclose the use of a process including methanol, hydrogen peroxide and deionized water in the passivation of mercury cadmium telluride electronic devices Hayes et al, U.S. Pat. No. 4,640,719, disclose a method for reducing rosin soldering flux from printed circuit and wiring boards by using a dish washing type cleaning system and a liquid composition comprising terpene compounds.
Similarly, Futch et al, U.S. Pat. No. 4,934,391, disclose methods and compositions for removal of soldering flux, screen inks and resists using compositions containing dibasic acid esters and surfactants. Mombrun et al, U.S. Pat. No. 4,983,224, also disclose methods of removing soldering flux using nonpolar liquid compositions comprising terpenes, terpenols, mixed with polar aprotic solvents and surfactants.
Ogaya, PUJPA 63-239820 discloses a method for cleaning electronic device assemblies using a spray of pressurized carbon dioxide. Both IBM Technical Disclosure Bulletins Volume 17, No. 8, January 1975 and Volume 19, No. 10, March 1977, disclose a mechanical cleaning process using deionized water and chelating agents to remove micro particulates. However, the disclosed application of IBM will not tolerate an acid based spray or provide the necessary sensitivity to avoid erosion or destruction during processing of the submicron conductors on those devices. Smith, Corrosion Resistance of Lead, Lead Magazine, page 511-522, discusses the corrosion resistance of lead when subjected to various environmental stresses.
However, these systems, although excellent for removing rosins and activators, are not preferred in cleaning for environmental and safety reasons. Further chlorinated solvents remove rosins but also tend to remove polar activators. Combinations of alcohols and chlorinated solvents have also been used but can contribute to the presence of fluorocarbons into the environment.